The present invention relates to a system for detoxifying heavy metals contained in sludges, soils, incinerated ashes and similar materials; being more particularly directed to a novel system for destroying potentially toxic volatile organics and solvents and for effecting the encapsulation, fixing and stabilizing of remaining residues to render them safe (by at least Environmental Protection Agency (EPA) standards) from the leaching out of, or latter exposure to, heavy metal products therein, such that the residues may be directly used in landfills or incorporated into building materials like concrete or asphalt, if desired.
Previous systems for the solution of the above problem have been made through the use of chemical fixation and stabilization techniques, including the combining of silicates and Portland Cement to produce a stabilized, solidified material. While such systems have had some effectiveness in binding heavy metals, the effectiveness in stabilizing volatile organics is questionable, and previous chemical stabilization processes unfortunately require increasing the volume of the sludge and/or ash by ten to twenty percent. The chemical solidified product may be landfilled or may be used for landfill cover material. The later-described fixed product of the present invention provides for about eighty seven percent less weight and seventy percent less volume, in some tests, and the volatile organics are destroyed.
The invention broadly comprises a system for volatilizing, fixing, stabilizing and effectively carbon bonding heavy metals in a metal-containing sludge, soil, ash or similar material that is not subject to the above-described disadvantages of prior chemical fixing and other techniques, but provides vastly improved, less costly and more efficacious stabilizing not only of the resulting residue but of the volatile organics and solvents driven off in the process as well.
The invention embodies a system that is uniquely adapted for carrying out one of the processes described in my parent application. Specifically, the system processes a metal-containing feed material, such as a mixture of 20 to 65% solids sewage sludge cake and inorganic residues. The wet feed is dried in an oxygen starved drying zone and then stabilized in an oxygen starved zone in which a char residue is formed and the volatile organic compounds are driven off. The heavy metals present are bound with carbon in the char residue and the volatile organic compounds are further incinerated in an afterburner. The resulting char residue has less volume than the original sludge or composite feed.
More particularly, the system provides a multizone starved air combustion unit capable of controlling temperature and residence time. The resulting residue contains an acid-insoluble char matrix. About twenty to fifty percent more residue is produced than with incineration, but the residue passes EPA's Extraction Procedure Tests for leaching heavy metals and is therefore well-suited for landfill disposal, or used as a light-weight aggregate. Heavy metals and other potentially toxic compounds are stabilized at temperatures below their volatilization temperature and below the temperature often used in other incineration or pyrolysis processes that do not produce a fixed residue. The thermal residence time is controlled to produce minimum weight without significant decarbonization (the latter being causable by excess oxygen). Toxic, volatile off-gas and odors are destroyed within a high temperature afterburner designed for appropriate residence time and temperature that will destroy the volatiles found in a particular metal-containing feed material.
While sludge incinerators and trash incinerators often produce ash residues containing toxic, leachable metals in their bottom ash and/or fly ash, the residue produced with the system of the present invention does not contain significantly soluble metals or metal oxides (e.g., chromium). The residue, furthermore, can be mixed with ten to fifteen percent, for example, of Portland Cement (on a weight basis), if desired, with the quantity of water equal to the weight of the residue, and some coarse sand. The resulting cemented residue material has a specific gravity of about one, it is fireproof, and it has sufficient structural strength to be safely used for lightweight building products.
Another illustrative application of the system is for stabilizing toxic metals within waste sludges from steel drum reconditioning plants, the empty drums containing a variety of residual materials which often contain toxic matallic and organic compounds. The resulting residues show a fifty percent reduction in volume with the toxic metals immobilized with carbon, and the solvents destroyed in the afterburner. The invention can also be used to process toxic waste and eliminate the need of disposing of these materials in hazardous waste landfills; with the residue used for landfill or solidified for use in building products such as cinder blocks, asphalt, concrete or other lightweight products, as before suggested.
As still another use, the system may aid in the recovery of chromium from the leather and metal plating industries since the char residue passes the EPA Extraction Procedure, indicating that the residue will not leach excessive quantities of chromium. Subsequent roasting of such residues containing chromium produces a chromate ash from which chromium can be extracted with the remaining insoluble residues treated by the invention to produce a non-toxic residue.